Method of making phosphor screen for color cathode ray tube

ABSTRACT

A METHOD OF MAKING A PHOSPHOR SCREEN OF A MULTICOLOR CATHODE RAY TUBE WITH COMPRISES THE STEPS OF COATING THE INNER FACE OF THE FACEPLATE OF THE CATHODE RAY TUBE WITH A PHOTOSENSITIVE ADHESIVE BINDER DEPOSTING A POSITIVE PHOTOSENSITIVE LAYER OVER THE ENTIRE AREA OF THE COATED PHOTOSENSITIVE ASHESIVE BINDER, DRYING THE DEPOSITED PHOTORESIST, EXPOSING THE PHOTORESIST LAYER TO IRRADIATION BY LIGHT THROUGH A MASK HAVING A PREDETERMINED PATTERN TO EXPOSE THE UNDERLYING PHOTOSENSITIVE ADHESIVE BINDER LAYER AT PREDETERMINED AREAS, DEVELOPING THE EXPOSED INNER FACE OF   THE FACEPLATE TO REMOVE THE PHOTORESIST LAYER FROM THE PHOTOSENSITIVE ADHESIVE BINDER LAYER TO EXPOSE IT AT THOSE AREAS EXPOSED TO LIGHT, RINSING THE DEVELOPED INNER FACE OF THE FACEPLATE WITH WATER, AND DUSTING PHOSPHOR OF A DESIRED COLOR ON THE EXPOSED AREAS OF THE PHOTOSENSITIVE ADHESIVE BINDER LAYER. THE PHOTOSENSITIVE ADHESIVE BINDER IS INSOLULBE IN WATER AND IN A DEVELOPER AFTER BEING EXPOSED TO LIGHT AND BECOMES STICKY DUE TO ABSORPTION OF WATER.

Unted States Patent O U.S. Cl. 96-36.1 9 Claims ABSTRACT OF THE DISCLOSURE A method of making a phosphor screen of a multicolor cathode ray tube which comprises the steps of coating the inner face of the faceplate of the cathode ray tube with a photosensitive adhesive binder depositing a positive photosensitive layer over the entire area of the coated photosensitive adhesive binder, drying the deposited photoresist, exposing the photoresist layer to irradiation by light through a mask having a predetermined pattern to expose the underlying photosensitive adhesive binder layer at predetermined areas, developing the exposed inner face of the faceplate to remove the photoresist layer from the photosensitive adhesive binder layer to expose it at those areas exposed to light, rinsing the developed inner face of the faceplate with water, and dusting phosphor of a desired color on the exposed areas of the photosensitive adhesive binder layer. The photosensitive adhesive binder is insoluble in water and in a developer after being exposed to light and becomes sticky due to absorption of water.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method of making a phosphor screen for multi-color cathode ray tubes.

Description of the prior art Heretofore, the so-called slurry method, dusting method and so on have been well-known in the art for the manufacture of the phosphor screen for multi-color cathode ray tubes.

In accordance with the slurry method a phosphor slurry comprising a predetermined color emissive phosphor dispersed in a photosensitive adhesive binder material composed of polyvinyl alcohol and ammonium dichromate is coated on the interior surface of the faceplate of a cathode ray tube and the coated surface is subjected to an exposure and a development process after being dried, thereby to fix the phosphor. The above processes are carried out for each color to provide a phosphor screen having a determined pattern.

With the dusting method, a photosensitive adhesive binder material is coated on the interior surface of the faceplate of the cathode ray tube and a phosphor is dusted on the coated surface while the binder material is still adhesive and then the coated surface is dried. Then it is subjected to an exposure and a development process, and this process is repeated for each color to provide a predetermined pattern on the inner surface of the faceplate.

However, both conventional methods involve coating the inner face of the faceplate with the photosensitive 3,677,757 Patented July 18, 1972 "tice adhesive binder or phosphor a plurality of times for each color and thus the coating processes for manufacturing the phosphor screen are complex and expensive.

Further, the use of a large-grained phosphor is desirable for enhancement of the brightness of the phosphor but in the slurry method dispersion of the phosphor in the photosensitive adhesive binder imposes a limitation on the size of the phosphor particles that can be employed. In addition, the slurry method uses ammonium dichromate in an appreciably large amount relative to polyvinyl alcohol (usually 40 to 60 parts by weight of ammonium dichromate with respect 1,000 parts by weight of polyvinyl alcohol), which raises deterioration of the phosphors by chromium ions.

With the dusting method it is difficult to maintain the uniform adhesive property of the photosensitive adhesive binder and this imposes a severe limitation on the manufacturing conditions. Further, during the exposure process after drying the phosphor, light is scattered by the phosphor particles which are mixed in the photosensitive adhesive binder and this lowers the resulting resolution and weakens the intensity of light reaching the inside of the coated layer. Also the reaction is not completed at the boundary plane of the faceplate, so that the ability of the phosphor layer to stick must be kept within certain limits.

SUMMARY OF THE INVENTION This invention eliminates the aforementioned drawbacks experienced in the prior art and has for its object a method of making a multi-color phosphor screen for a color cathode ray tube which comprises the steps of coating the interior surface of the faceplate of the color cathode ray tube with a photosensitive adhesive binder which becomes insoluble in water and a developer and sticky by absorption of moisture after exposure, depositing on the coated photosensitive adhesive binder positive photoresist over its entire area to a thickness suicient to permit light to pass during the exposure process, exposing the positive photoresist layer and the photosensitive adhesive binder lying thereunder to irradiation by light at predetermined areas through a mask after drying the positive photoresist layer, developing the coated interior surface of the faceplate to selectively remove the photoresist layer from the photosensitive adhesive binder at those areas exposed to light, rinsing the coated surface with water to cause the exposed areas ofthe photosensitive adhesive binder to have suicient adhesion for phosphor and dusting desired phosphors on the exposed areas.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. A to E are diagrammatic views of the steps involved in the manufacture of a phosphor screen in accordance with one example of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. A to E reference numeral 1 indicates a faceplate of, for example, a color cathode ray tube, the inner face of which is coated with a photosensitive adhesive binder 2 composed of, for example, polyvinyl alcohol and ammonium dichromate to form a photosensitive adhesive binder layer 3.

The photosensitive adhesive binder 2 is rendered insoluble in water and a developer by exposure to light and adhesive enough by absorption of water to permit adhesion of phosphors thereto. The photosensitive adhesive binder is of a type such that ammonium dichromate contained therein is reduced by light to produce ions Cri and the resulting ions Craf and polyvinyl alcohol are cross-linked together so they are insoluble in water and in the developer. The unreacted polyvinyl alcohol is sticky. With conventional methods, the unreacted polyvinyl alcohol causes mixing of the colors and complete reaction of the polyvinyl alcohol causes an increase in the amount of remaining chromium ions which results in the reduction of the brightness of the phosphors. With the present invention, however, the amount of ammonium dichromate added to the photosensitive adhesive binder is selected to be 1,420 to 3&5 of the usual one, and about 1.6 to 3.0 parts by weight of ammonium dichromate is added to 1,000 parts by weight of polyvinyl alcohol to cause complete reaction of chromium which gives the photosensitive adhesive *binder in above-mentioned desired properties. Alternatively, the blending ratio of the materials is the usual one or exceed it and the time of exposure is shortened to control the aforementioned crosslinking reaction, thereby to give the photosensitive adhesive binder the properties mentioned above.

After the formation of the photosensitive adhesive binder layer 3, a positive photoresist layer 4 is deposited on the binder layer 3 as depicted in FIG. B. The positive photoresist is of the type that it becomes soluble in a developer when exposed to light. The photoresist, may be type FHR-l00, FHR-700 (Trademarks) produced by Fuji Yakuhin Kogyo Kabushikikaisha. The positive photoresist is coated on the binder layer 3 to a thickness of about l to 20 microns which permits the passage of light therethrough. After being dried, the coated inner face is exposed to irradiation by light through a mask P having a predetermined transparent pattern at those areas which will be ultimately occupied by a phosphor of a desired color, as shown in FIG. C. The positive photoresist layer 4 is so thin that the underlying photosensitive adhesive binder layer 3 is adequately exposed to light. In the event that the amount of ammonium dichromate contained in the photosensitive adhesive binder is 1/20 to 1&5 of the usual one, the time of exposure becomes a little longer but the time of exposure in the process depicted in FIG. C can be shortened by exposing the entire area of the photosensitive adhesive binder layer 3 to light to incompletely harden it prior to the formation of the positive photoresist layer 4. Then, the coated inner face is subjected to a developing process using a positive photoresist developer (such as manufactured by Fuji Yakuhin Kogyo Kabushikikaisha) to selectively dissolve the positive photoresist layer 4 at those areas S exposed to light, thus exposing the photosensitive adhesive binder layer 3 underlying it, as depicted in FIG. D. Thereafter, the coated inner face is rinsed with water to wash away the developer remaining thereon, leaving void areas in the positive photoresist layer 4 to expose the underlaying photosensitive adhesive binder layer 3 at selected areas as indicated by 6 in FIG. D. The exposed areas 6 of the photosensitive adhesive binder 2 absonb water and become adhesive and sticky. Then a phosphor 7 of a desired color is dusted on the rinsed inner face over its entire area, by which the phosphor 7 gets mixed into the exposed photosensitive sticky adhesive binder layer 6 as shown in FIG. E. The unexposed surface of the positive photoresist layer 4 is relatively smooth, so that the phosphor is blown off by the air pressure during spraying and does not adhere to the surface of the positive photoresist layer 4.

The above processes shown in FIGS. C to E are repeated for mixing phosphors of other colors into the photosensitive adhesive binder layer 3 to form a three color tube, for example. This method is also applicable to making tubes with the so-called black matrix between 4 the color phosphors by placing a material such as manganese dioxide MnO2 or the like which become black when heated at those areas of the screen on which the color phosphors are not deposited. This improves contrast. An improved phosphor screen of a multi-color cathode ray tube is thus produced.

As will be understood from the foregoing, this invention requires that the photosensitive adhesive binder be coated only once and hence simplifies the manufacturing operation and, at the same time, the thickness of the phosphor layer can be easily changed by suitably selecting the amount of ammonium dichromate added to the polyvinyl alcohol to make up the photosensitive adhesive binder and the time of exposure. Further, the photosensitive adhesive binder layer is temporarily covered with the positive photoresist and this makes it possible to hold the stickiness of the photosensitive adhesive binder layer uniform after exposure.

In addition, this invention provides for enhanced resolution and ensures the avoidance of mixing of colors and further allows the use of large-grained phosphor to improve brightness.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

I claim as my invention:

1. A method of making a phosphor screen of a multicolor cathode ray tube comprising the steps of:

coating the inner face of the faceplate of the cathode ray tube with a photosensitive adhesive binder which is insoluble in water and in a developer after being exposed to light and sticky due to absorption of water;

depositing a positive photoresist layer over the entire area of the coated photosensitive adhesive binder; drying the deposited positive photoresist;

exposing the positive photoresist layer to irradiation by light through a mask having a predetermined pattern to expose the underlying photosensitive adhesive binder layer at predetermined areas;

developing the exposed inner face of the faceplate -to remove the positive photoresist layer from the photosensitive adhesive binder layer to uncover it at those areas which were exposed to light;

rinsing the developed inner face of the faceplate with water; and

dusting phosphor of a desired color on the uncovered areas of the photosensitive adhesive binder layer.

2. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 1 wherein the photosensitive adhesive binder is composed of polyvinyl alcohol and ammonium dichromate.

3. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 1 wherein the positive photoresist layer is thin enough to permit the passage therethrough of light during the exposure step.

4. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 2 wherein the photosensitive adhesive binder contains 1.6 to 3.0 parts by weight of ammonium dichromate relative to 1,000 parts by weight of polyvinyl alcohol.

5. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 2 wherein the photosensitive adhesive binder consists of 40' to 60 parts by weight of ammonium dichromate relative to 1,000 parts by weight of polyvinyl alcohol to shorten the time of exposure.

6. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 1 wherein the phosphor is either red, green or blue phosphors.

7. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim l wherein after the phosphors have been deposited a material which becomes black when heated is placed on the inner face of the faceplate of the cathode ray tube at those areas on which the color phosphors are not deposited.

8. A method of making a phosphor screen of a multicolor cathode ray tube as claimed in claim 7, wherein the black material is manganese dioxide.

9. A method of making a phosphor screen of a multicolor cathode ray tube according to claim 1 comprising the steps of exposing the positive photoresist layer to irradiation by light through a second mask having a pattern to expose the photosensitive adhesive binder layer at second predetermined areas, developing the exposed inner face of the faceplate to remove the positive photoresist layer from the adhesive binder layer at said second predetermined areas, rinsing the inner face of said face- References Cited UNITED STATES PATENTS 3/1966 Johnston 96-36.2 1/1967 Spiers 96-36.2

10 WILLIAM L. IARVIS, Primary Examiner U.S. C1. X.R. 117-335 CM 

